Study of calmodulin (CM-regulated cyclic nucleotide phosphodiesterase (PDE) reveals that (i) deactivation of the CM-activated PDE is achieved by the removal of one Ca(II) from the Ca(II).CM.PDE complex. This is consistent with the proposal that Ca(II)4.CM,.PDE is the active species; (ii) the existence of Ca(II)n.C.PDE where n is 1, 2, 3, and 4; (iii) Ca(II) dissociation from Ca(II)4.CM and Ca(II)4.CM.PDE proceeds via multiphasic exponential function, and complexation with PDE causes each Ca(II) off-rate by a factor of about 100. (2) A fluorescent peptide has been synthesized. This peptide is shown to be a good substrate for cAMP-dependent protein kiniase and its phosphorylated form serves as a substrate for phosphoprotein phosphatase. (3) Theoretical analysis of cyclic cascade model was extended to a branched cascade system, the HMG CoA reductase cascade. (4) Study of a spin-labeled Tempo-adenylylated glutamine synthetase revealed changes in distances between the adenylyl site and the two metal ion sites due to substrate binding and formation of an intermediate. (5) Kinetics and thermodynamics of divalent metal ion binding to a Ca(II) specific indicator were studies. The mechanism of a actomyosin ATPase was investigated.